Malaysian investigators are checking the identity of four passengers on a missing Malaysia Airlines flight, but have not yet determined if the plane was downed by an attack, the country's transport minister said on Sunday.

"All the four names are with me," Hishamuddin Hussein, who is also defence minister, told reporters.

A six-country search off the Vietnamese coast for the missing Boeing 777-200ER, the 227 passengers and 12 crew on-board yielded nothing so far aside from sighting of an oil slick, which Hussein has confirmed.